Phenocrysts in silicic volcanic rocks in the northern part of the Okhotsk-Chukotka Belt and their crystallization conditions

Abstract

Microprobe analyses of phenocrysts in silicic volcanic rocks from the northern part of the Okhotsk-Chukotka Volcanic Belt are reported. The parameters of the state of the magmatic system evaluated using techniques of mineralogical thermobarometry indicate that the phenocrysts crystallized at 750–1050°C. The most realistic results are likely yielded by the QUILF program package and the amphibole-plagioclase equilibrium. The calculated temperatures are sometimes higher than the average estimates made for other volcanic areas worldwide with the use of analogous mineral assemblages. The likely explanation of this is a relatively low water pressure in the parental silicic magmas of the Okhotsk-Chukotka Volcanic Belt (OCVB), which, in turn, suggests that the protolith did not contain much hydrous phases. The lithostatic pressure under which the amphibole-bearing assemblages crystallized was evaluated at 1–4 kbar, which corresponds to depths of 4–15 km. The Mg# of the biotite and amphibole exhibits obvious spatial variations, likely due to variations in the oxygen regime in the feeding magmatic chambers. The $f_{O_2 }$ level calculated for the frontal zone of the Central Chukotka Segment (CCS) varies between NNO + 1 and NNO + 3, and those from Anadyr Segment (AS) and the rear zone of the Central Chukotka Segment (CCS) are between QFM − 1 and NNO + 1. The possible factors that most efficiently controlled the oxygen fugacity in the silicic magmas seem to have been variations in the contents of highly reduced phases (such as organic carbon) in the crustal protolith. At a relatively homogeneous bulk composition of the silicic volcanics, the compositions of phenocrysts of Fe-Mg silicates can significantly vary, and this makes it possible to utilize these phenocrysts as an additional tool for the differentiation and correlation of volcanic sequences.